Method of and apparatus fob heat



y 1943- E. MITTELMANN 2,324,525

METHOD OF AND APPARATUS FOR HEAT TREATING METAL Filed March 22, 1941 2 Sheets-Sheet l 1 POWER 2 Z ABSOKBED Z OZ CUR/5 Pal/v7 u I 7':M R,47w/2 IMPEDANCE TEMPEEHTUIEE TIME SOURCE F a F TO LEA D 47 4 TO CONTACT 43 C l RCUIT Z zc-de w 261K" Zzzyezo fill 'ZZelmzmlt July 20, 1943. E. MITTELMANN 2,324,525

METMOD OF AND APPARATUS FOR HEAT TREATING METAL 2 Sheets-Sheet 2 Filed March 22, 1941 Index) Z01! Hurt-FUN QP ftu'b'e eircuit for effecting a complete control over ,the heat-treating operations, thereby eliminating the expense of motor generator equipment and .iixtures and also eliminating time necessary to. compile operationaldata and the Waste in- "valved in" operating the motor-generator or spark-gap type of induction device, or any other Patented July 20, 1943 UNITED STATES PATENT OFFICE METHOD OF AND APPARATUS FOR HEAT TREATING METAL I Eugen Mittelmann, Chicago, Ill. Application March 22, 1941, Serial No. 384,662 15 Claims. (01. 219-13) ing'and more particularly to a method -of and apparatus ior the controlled heat treatment of been proposed that sparkelectric'generator, sets to supply the power to the spark-gap has been practically non-existent. The reason for this is that the spark-gap generator represents a low impedance source, one

. hich will'notvary in regulation characteristic ivvlth variations of the impedance of the load. Hence, in the common use of such induction .heating .inethod's, the amount of heat applicationis entirely empirically determined. This, of

course, requires compilation of charts, tables, graphs and the like which must be experimentally m'ade'1"o1 each particular piece of material \treated. g

In my new method and means I utilize the "tional characteristics of a simple vacuumniotor jgenerator apparatus.

' It is the primary object of this invention to provide anovel andimproved method of and meansfor the heat treatment of metals by means of electrical induction.

' A. further object of this invention is to provide a novel and improved method of and means for l'heat treating-metals which will enable the heat treatment to be controlled at alltimes.

A further object of this invention is to provide' novel and improved apparatus for heat treating metal which will comprise a suitably ar- "ranged electronic circuit with means permitting automatic'control of any one or more of a number of desired operations.

A further objectof this invention is to provide *a heat treating method and means which will regulate and control the, heating of the metal automatically by reason of the influence of the fim'agnetic and physicalproperties of the metal.

An important object of this invention is to This invention relates to the art of heat treatprovide a novel method of and means for the heat treatment of a metal member by high frequen'cy induction wherein substantial amounts of power, as desired, may be induced into the metal member, regardless of permeability changes thereof, without in any way disturbing operating conditions existing between the'high frequency generator utilized and the source of power for said generator.

A further object of the invention is to provide a novel method of and means for the heat treatment of a metal member by high-frequency induction Wherein,when the rise in temperature of the metal member has changed the impedance of the generator load from its initial value to another given value, a 'rematching of the generator to the load will automatically occur.

Still a further object of the present invention is to provide a method and means for the heat treatment of metalsrwhich will utilize a'definite relationship between the operational characteristics of the heating device and the magnetic and physical behavior of the metal for the purpose of controlling the heat treatment.

f Other objects of the invention reside in prolv'iding' various automatic control means for the protection of the device, for the feeding of the metal into the operating portion of the device, for

the maintenance of the matched impedance in various portions of the device, for the sequential operation of various portions of the device, and for the operation and discontinuance of operation of still other portions of the device, all of said automatic control means being dependent h for operation upon the operational characteristics of the device having a definite relationship with the behavior of the metal While undergoing the heat treatment.

With the foregoing and other objects in View which will appear as the description proceeds,

the invention consists of certain novel features 7 of construction, arrangement and combination of electrical and mechanical parts and elements, procedures, modes and steps of practice, hereinafter fully described, symbolically illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that the various changes in the form, proportion, size and minor details and arrangements of the various elements of the apparatus and in the details of procedure of the method may be made without departing from the spirit or sacrificing anyof the advantages of themvention.

For the purpose of facilitating an understanding of my invention, I have illustrated in the accompanying drawings a preferred embodiment of the apparatus thereof and data explanatory of the operational characteristics and principles involved, from an inspection and examination of which, when considered in connection with the following description, my invention, its mode of construction, assembly, operation, arrangement of the elements thereof, the mode of practicing same, and many of its advantages andfeatures should be readily understood and appreciated.

Referring to the drawings in which the same characters of reference are employed to indicate the corresponding or similar parts'throu'ghout the several figures of the drawings:

Fig. 1 is a graph indicating various characteristics of the apparatus of my invention and or a member or metal treatedby means of my Fig.3 and Fig. 4'are detailed portions of'cir- 'cu'i'ts of the apparatus of my invention, showing modifications thereof. I

In general my invention contemplates :a highfrequency generator, the regulation character istic' 'ofwhich will vary in' accordance with the diately thereafter rapidly drops along the line P3 to a constant value represented by P4. The point P5 at which the power absorption drops to its lower level is known as the Curie point. or the point of magnetic transformation. At the Curie point, the magnetic permeability of the metal becomes unity and the field strength is a minimum;

The alteration of field strength, of course, has

its effect upon the characteristics of the electrical circuit causing the heating. That effect will be manifest in alteration of the impedance of the coil surrounding the metal piece and serving to induce theeddy currents therein. Disregarding the generator and looking into the heating. coil circuit itself, electrically, it will be obvious that as a result of the power being absorbed, 'theimpedance of the coil circuit will vary in accordance with the power absorbed, the impedance being about inversely proportional to the power. Thus thejcurve of the graph of Fig.

l which represents impedance of the coil circuit or Ioad'may'berepresented by Z as being the "reverse of the'power absorption curve P.

behavior of the load under operating conditions."

Knowing that the behavior of the generator circuitor device represents various conditions occuring in the piece of metal, for example, that is being treated, I'uti1ize those operational be haviorisms to operate various control means to "perform certainbther operations'fsuch as for example, turning off the generator, quenching the metal piece, altering thematching impedanc'efof thefcirc'uit, and the like. I

'It'should be appreciateibefore actually discussing my improved and preferred apparatus in "detail, that it may be possible for a motor-generator' device to be constructeldin which the "regulation will v ry with'the'load to' a certain degree and in which case some degree of regulation or controlma'y be possible. For complete flexibility and control;however,'I.utilize a vacu- "um tube'circuit' which I shall presently describe and which lends itself 'tofacbolflplishment Of the j features which are 'part of this invention in a "liighIy. advantageous and eflicient' manner.

When a metal rnembr is disposed within a "magnetic field, as forxample' by placing same 'l'thin a coil energized by electrical current,

ere'is a threadingof' the metal by magnetic mix as the well kriownmanner. If the elec-.

than current is alternating, the flux will be constantly changing, resulting in eddy currents through'the 'metal which, because'of their being short circuited, serve to ,heatfthe metal. The

intensity of heat'produced by these currents depe'nds' ,ob'v'io'ush upon the strength, of the magneticj field, the frequency of the current, and "the" magnetic permeability of the metal itself.

."Iheme'tal will increase in temperature because of the losses resulting-from the heating in acoordancewith-a certain well known lawor principle which is represented in the graph of Fig.

1;. -Atten tion is directed to the line marked P .which represents the power absorbed by the metal piece within the coil, under any given set of :point. If no -power factor correcting means is Lused in the line, the cost of such practice is very high. Other methods employ shielding and The impe'dance of the load is seen to suddenly jump to a maximum after the Curie point has been reached and'passed. The metal member "absorbspower along the line'Pd at a steady rate of a'considerably reduced value because of the "sudden increase In loadimpedance. For any *Ti'ilntil the'Curie point at'which thesud den change of power absorbed and impedance causes a sudden change in therate of temperature increase. Then'the' temperature rises slowly, tafpering oif alonga line T2 representing temperatu'rescorresponding to the power'absorbed along the line Pi. The temperature of the metal "member may, under certain conditions, drop considerably after the' Curie point has been reached and continue to decrease if the radiation losses consumemore power than can be absorbed by the member from the generator. In any event, it is obvious that in order to raise thetmperat'ure or the metal member to any appreciable degree above that attained at the Curiejpoint, it will be'necessary to cause'the metal member to continue to absorb substantially the same quantity or power as it had been absorbing 'up'to the time that the Curie point was reached.

For steel working processes requiring high temperatures, the time. within which the piece must reach therequired'temperature is'so short as to "make temperature increase along a line T: im-

practical and uneconomical, if not impossible.

.-It has been the practice in inductlon'heatlllz processes to increase the generator power input immediately that the, Curie point has been reached and'force' power absorption along a line parallel to Pr but above it. This is done by increasing the excitation'of the'fields'of the macrease of ampere turns serves to draw a much greater reactive component of current from the chines supplying the electrical power. The inline than the already greatly reactive component being drawn prior to the reaching of'the Curie auxiliary heating means-to raise thetemperature of the member. The temperature control pos- I an rm-mnunhsnmmmhl swi control at all times.

1 has been reached.

sible by these methods is more or less empirical and the cost very high in comparison to that of my'new method.

-.In my invention, the entire process is under The generator which I use is provided with means comprising one or more control-circuits in which the characteristic Z and P curves ofFig. -1 are reflected as electrical prop.- erties of said circuit or circuits. I provide means to beactuated by said circuitor circuits at any time or times with respect to the electrical and physical condition of the metal member as reflectedin the control circuit Therefore, any chosen point or points of the P curve, for ex- :ample, may be chosen for the operation of said means. The means employed may cut off the namely of supplying more power to the metal member after the Curie point has been reached,

"is economic'ally solved in a simple manner by my invention. As previously mentioned, the sparkgap or motor-generator type of generators have low impedance resulting in substantially no change in generator impedance for change in load impedance. Now, it is well known thatthe greatest transfer of energy from a feeding circuit -to a fed circuit occurs when the impedance looking into each circuit is the same.

then, this type of inflexible generator operates at Obviously,

a'low efiiciencysince under no practical conditions is the generator ever matched to the load, whether operating at normal power input prior to reaching the Curie point or whether operating at increased power input after the Curie point Inmy invention, the load is matched with the generator at the start of the process; so that 1 a maximum of energy can be transferred to the piece; "After the Curie point is reached, the impedance of the load'has jumped, a control circuit automatically re'matches the generator to "the'load, thereby permitting a greater power to be forced into the member at substantially the same rate that'it was being absorbed prior to the Curie point being reached. The temperature of Lthe metal member "can thus be simply and quickly raised, as'shown in Fig. 1, along the broken line T3, to any desired value. During the entire process the generator input has remained at practically the samelow value as at initial "conditions; there have been no changes in opapparatus and methods.

my invention I use a novel vacuum-tube 'oscillatonto supply the high-frequency power necessary to treat the metal, The tubes of the oscillator are grid-biased so that they operate .essentially as class C oscillators on the curved portion of the Ip Eg curve. Now, by alteration.

of the factor of merit Q of the coil of the tank circuit which is done by increasing the loss of the coil, that is, the power absorbed by the metal piece, there must be a corresponding alteration in the plate currentof the oscillator. As the Q, of

. ,th e tank circuit of theyoscillator decreases, the plate..current of the tubes will increase. This may be understood by visualizing the equivalent priate condenser Ci.

circuit of the oscillator tube which includes a relatively constant voltage. As the load is increased, the plate voltage remaining practically constant, the plate current must increase. The increase of plate current will vary with the load because tubes are being operated upon the curved portions of their characteristics.

Since the plate current of'the oscillator will vary with the load; it is obvious that the variation will closely follow the curve represented by P in Fig. 1 and will have portions corresponding to P1, P2, etc.; hence, for any given piece of metal being heated by the oscillator,.at the point that the plate current reaches the value corresponding to P5 of the curve P, the Curie point will have been reached and can be used, for example, as a point of discrimination in the operation of Various devices to cause an automatic cyclic control of the device. Obviously other points along the P curve could be so utilized.

Referring now to the circuit diagram shown in Fig. 2, I have utilized as the basis for my circuit a pair of high powered triodes l2, l2 designed to utilize a plate voltage of approximately 3000 volts supplied by an appropriate rectifying circuit and designed for a plate current of approximately one ampere maximum. This arrangement is intended to produce a power output of approximately two kilo-watts when the tubes l2 are used in the well known push-pull arrangement and thecir'cuit constants are adjusted for a frequency of approximately one megacycle.

The plates 1! of the tubes [2 are connected to the opposite ends of the primary coil L1 of the tank circuit which is in paralle1 with an appro- Tapped off from the primary coil L1 at appropriate positions are a pair the metal member. electrical properties of. coil and member will,

of leads [3 which connect to opposite ends of the heating coil L2 within'which the metal piece i0 is adapted to be positioned for the heat treating operation. 1

This arrangement is a novel and advantageous one and permits the operation of my apparatus at its maximum efficiency, as will presently be explained.

In any heater coil associated with high-frequency induction heating apparatus, the coil, together With the metal member within it, represents a certain load impedance, the magnitude and phaseangl'e of which depends upon the .dimensions of the coil, upon'the geometrical and electrical relation of the coil to the metal member within it, andupon various other factors, all of which affect the coupling of the coil with The interdependence of the therefore, cause varying load impedances due to changes in thephysicaland electrical state of the metal member while being heated.

The practice has heretofore been to connect theheater coil parallel to the tank circuit of the generator. The coil is thus a frequency-determining part of the oscillatory or tank circuit. This has even been the practice where it has been proposed using thermionic oscillators. The internal impedance of thermionic generators is of ahigher order than that of loads comprising coils and members, and hence the energy transfer is very inefficient dueto the poor power factor. The maximum amount of energy transfer is therefore dependent upon the size of the capacitance in the circuit since the heater coil is parallel therewith.

As will be noted from the diagram of Fig. 2, my

a. V V I 2,824,525

heater coil lo is notlconnected :parallel with the tank circuit,-but.rather'is connected witha'fraction ofthe' total impedance of thetank circuit inductance L1. Thus theheatercoilLa andmem- 'ber l0, together. constituting'the load, .-may be This is possible since the center tap-I4, correaspond n the positive. terminal ofithe high ten sion-plate supply circuit, is grounded. 7

With the above described couplingythe tank circuit is always at resonance and hence the tubes .fl-zare always operating at their maximum efficiency. The frequency of the tank circuit will .varybut slightly, and the tank circuit presents essentially a substantiallypure ohmic resistance to the passage of plate current. Since there- .sistance varies as the-load, that is, heater L1 and member 10, the plate'current will vary as the load and give the desired-reactions in the control circuits 44 and I4. In the absence of the arrangement described, the heater coil would .iorm part of the frequency'determining tank .circuit, and the plate current would depend primariiyu on the ratio of -L/C in the. tank circuit. 'If L were sufficiently small, variations of additional load resistance, as'causedbyincrease in temperature of the member lowould'not -appre ciably influence the plate current, thus failing .under these conditions, to give'the desired refie'ct'ed variation of plate. current permitting use ,of the control circuits 44 and. M in the manners described.

The center tap l6 of theprimary coil Ll is the plate lead and connects with an appropriatesupply of plate voltage which is representedas a .recti er 15. The lead I4 has a plate current meter Is in the circuit and also the coil-of the relay R1 positioned to receiveplate current; The feed-back leads-I1 are connected'to'taps on oppositev sides of the midpoint of the'tank primary coil L1 andlead to the grids [8 of the tubes [2' through appropriate condensers C2. Thisis, .of

course, a conventional connection and. serves to "feed to each tube "a voltage which is 180 out offphase with the impressed voltage thereby'promoting the oscillation of the tubes; 1 I

The negative'terrninal of the rectifier l leads to the cathodes through an appropriate cathode resistor 29 which is variable'in' order to "enable an appropriate bias to be placed upon the grids ll! of the tubes. The negative end of the cathode resistor is also connected to'the two grids I8.

' through the center tap of the grid leak resistance 2 l' which also aids in biasing the'grids. The grid leak resistor 21 in cooperation with the-con- "densers C2 serves to cause the oscillator tobe self-starting and is usual in class c. operating oscillators. I

..In order to more clearlyhexplain theoperation "of the. device, the various portions of the circuit will be explained as the description:of: the opera- -.tion requires.

.Assume that a piece of metal lfl has :just been inserted-in the coil 12 andthe oscillatoris heating same. By previous adjustment of theshtmt 2| across the relayRi said relay may havebeen adjusted to be energized at the maximum current -.in theplate circuit l4 corresponding to the point 1P2:'0f .the'Pcurve. .This point P2 is inst before theaCurie .pointand when that point has been areached .it will-cause.the-contactsa22 and 113 .11) cengage, .closingthereby the. circuit of thereby R2 through the D. C. potential of the bridge rectifier.

. This circuit includesthe lead 25 within which are positioned the normally closedv contacts Z8 and'ZI whichxare. adapted to be separated at an appropriate time by the relayRi. The positive side of the bridge rectifier 24 isconnected to the relay R2 bythe lead 28.

It is noted that duringthis period the cut-off grid bias :resistor 29-15 short circuited throuflh the contacts 30 and 3t and the contacts 32 and ,33 associated with the relays R4 and R1 respectively which contacts are normally engaged.

Hence, any voltage which might appear across the resistor will be short circuited and the grids i8 are biased according to the adjustment set on the cathode resistor 20 in accordance with the desired relations to the prevailing load conditions. 7

.The alternating current poles of the bridge rectifier are'connected to the-secondary of a transformer 35, the primary of which is connected across the alternating current line 16 through the usual line switches. and the micro-switch 38-39. The rectified current is hence ,r d from the bridge rectifier 24, which inciden- -tally may be any type suitable to the equipment available, through the leads 25 and 28 to energize the relay R2. The energizing of this relay will attract the contacts 40 and M to ueen- 7 othensame being normally open thereby connecting the relay R2 directly to the D. Qpoles of therectifier 24 and holding same in saidconnection independent of therelay R1 and hence independent of the plate current in thdplate circuit l4. At the same time, the contacts 42 and 43 which-were heretofore separated will engage.

I All of'theabove occursalmost instantaneously and within the time that the value ofthe plate current has dropped from its maximum toits steady state condition represented on thecurv e 'P'bythe'Curie' point'Ps; In'other words; along the current curve, the above described operations have occurred on'the portion corresponding to P3 of, the P'curve, Y With the contacts 42 and 43 engagingfthe device is ready tor the operation of the mechanism designed to be actuated when the Curi point is reached.

When the Curie point-of magnetic transformation Ps'is-reached, the; plate current'in the coll'Lris at a minimum'inasmuch as the plate current follows theload. The circulating high frequency current in the tank circuit and coil L1 is a maximum at thispoint, and hencethe 1 current in a secondary coillhinductively positioned with respect to the'coilLr will be a max- -imum. This secondary coil L3 is connected in-a secondary circuit 44 which includes, besides the coil 10, 9. rectifying unit 45such'as a selenium or copper oxide IGCUfiBL: and a relay Ra. con- "nected in series therewith. A condenser: Cir-is --connected across the cricuit. to smooth out the ripple in: the direct current produced. The-n- A ne lay will serve'to attract the contacts 45 and 46.

Inasmuch as the contacts 42 and 43 are being held closed by the relay R2 the circuit from the rectifier will be closed for the relay R4, through the contact 46, lead", relay R4, lead 48 to the positive side of the rectifier, and from contact 45 through contacts 42 and 43, lead 25, and contacts 26' and 21 normally engaging, to the negative side of the rectifying bridge 24.

"Immediately that. the relay R4 is energized, it causes engagementbetween the contacts 49 and 50, thereby directly'connecting it to the bridge independently of the other relays of the circuit, separating the contacts 26 and 2'! and thereby releasing the relay R2, and also causing opening of the contacts 30 and 3 I.

The opening of the contacts 30 and 3| removes the Short circuit which existed around the resistor 29 through the leads 5|, 52, and the contact 32, 33. Thus the resistor 29 is now inserted into' the self-biasing cathode to grid circuit.

This,"of course, adds'considerably to the bias of the grids 18 which are already quite negative, and moves the-biasing voltage to a point so far pastthe cut-ofi grid voltage that the oscillationsof the tubes are halted and the heating of the metal piece is interrupted.

The last described series of reactions occurs I 7 almost instantaneously and in effect the power of the oscillator is cut on just after or at the moment that the Curie point is reached. It is singular-to note at this point that the time in which the metalis made to reach the Curie point is in the order of seconds. This is in connection with its use to treat saw-blades and small machine parts.

' The sudden opening of the circuit containing the resistor 29 causes a sudden transient surge or'impulse to be impressed upon the'primary ofthe transformer 55. One side of the secondary of the transformer 55 is connected to the grid-of the'gas-filled grid-controlled rectifier l impressed on the grid 56' of the rectifier 51 from the 'transformer 55 will cause a passage of current in the plate circuit SI and hence will energize' a relay R5 which is interposed in the plate circuit. 1 1

The energizing of the relay R5 will cause engagement of the contacts 62 and 63 which con- 'nects the line 36 across the coil of the plunger relay' R through the normally closed micros'witch 38-39, thereby energizing the plunger relay Raand causing movement of its plunger 64. =It should be recalled'that the drawing is 2. diagrammatic representation, hence relative directionsand mechanical details referred to are obvoiusly explanatory only. Movement of the plunger. 64.:will cause the sliding plate 65 which is''.attached thereto by the link 66 to move to the-left ascviewedin the drawing. The solid portion 61 of the plate 65, which had been 'arranged to support the piece III while same was disposed within the heating coil L2 moves out The from under the piece l0 and the slotted portion 68 of the plate 65 moves into registry with the bore of the heating coil L2 thereby causing the piece ID to drop through the plate 65 and into the waiting quenching bath 69. Any other method of automatic quenching by use of a plunger or other type relay is contemplated by --my invention, the above-described means being merely illustrative.

This last mentioned series of operations also occurs almost instantaneously so that the metal is quenched at the precise moment that it is cherry-red or any other desired color, or at any fraction of time thereafter as controlled by the responsive speed of the relays. 5

The final fraction of the movement of the plunger 64 is utilized in opening the contacts 38, 3901 the micro-switch which opens the line 36 and thereby cuts off the energizing current of any and all of the relays dependent thereon, thereby returning the entire system to normal condition poised for the next cycle of'operations.

In order to prevent the operation of the oscillator when there is no load in the heater coil L2, I provide a photo-electric system for opening the contacts 32 and 33." The source of light 10 directs a beam H across the path of the metal piece 10 at the cell and amplifier 12. When the beam 1| isinterrupted by a metal piece I0, no current flows through the relay R7 and the contacts 32 and 33 are closed. However, when the metal piece drops into the quenching bath 69 and out of the path of the beam H, the beam is directed onto the cell and amplifier 12 energizing relay R1 which opens the short circuit around the grid resistor 29 thereby cutting on the oscillationsof the tubes l2, l2 so that no power is generated thereby. This, of course, is done by removing the short circuit from the grid resistor 29 and thereby causing the voltage across same to be added to the grid bias thereby'biasing' the grids l8 so far past cut-off that no appreciable plate current flows. j

The above outlined procedure is entirely automatic, as will be obvious'from the explanation thereof, the only thing not entirely controlled by the behaviorism of the metal itself being the feeding of the metal pieces into the heater coil. This, however, is quite possible by the installation of an appropriate mechanism to be actuated for example by the piece which is falling into the quenching bath or by any other obvious means whereby a continuous unattended process results.

The above described apparatus has been described in detail merely to illustrate a manner in which my invention may be applied. The auxiliary circuit 44 in which the relay R3 may be made to operate at a point of high impedance of the load, and the plate circuit i4 containing the relay R1 which may be made to op crate at a point of low impedance of the load, are control circuits. The characteristic impedance curve Z is reflected in one control circuit and the charactristic power absorption curve is reflected in the other circuit, and said curves appear as current characteristics of the control circuits.

It thus appears that the control circuits may be made to perform any desired functions at practically any point or points with respect to to the P curve. The relaysRz and R4 aid 'in making the apparatus entirely automatic and are operated also by the control circuits. It is therefore obvious that functions which are dee sired can be accomplished directly by the control circuits.

For example, either of-therelays R1 or R3 could directly open the circuit shorting-out the resistor 29 at any desired point of the load characteristic; or either relay could serve to fire the gas filled triode 5I which could' move the member 65 to quench the piece ID at any given point ogthe P curve; or, the triode' may be made to do any other desired work. The important feature is that the control circuit may be made to operate electrical or'mechanical devices associated therewith upon reaching, or at a predeterminedtime after reaching, a given physical or electrical condition. r

If it is desired that temperatures higher than those obtained by heating the metal piece Hi to the Curie point of magnetic transformation be reached, a greater flow of energy to the metal piece will be required, as previously explained. Ithcnce will be necessary to change the impedance of the generator at that precise point, since, as. will be noted from graph Z of Fig. 1, the impedanceof the heating coil and its load is quite higher than under the initial conditions atwhich the generator impedance was substantially matched to that of the load. In case, insteadof utilizing therelay Ra only for the purpose of operating other'relays to cut oil the oscillating tubes, relay R3 is arranged to also switch the leads of the coil L2 to a greater number of turns as shown in Fig. 3 at the Curie point. In this manner, the heating coil L2 and its load impedance are rematched to the tank circuit and its impedance. This. is'required in order toforcepower to be absorbed by the metal piece II. The contacts 'l3-in leads l3 are switched by means of the relay R of the secondary circuit 44 fromthe low number of turns connected by means of the leads 114 to the higher number of turns connected to-the coil Lr by the-leads 15. a

In order to determine how long the metal piece is to be subject to this greater influx of power, ameasurement of the high frequency power being absorbed maybe made by means of a high-frequency wattmeter suchv as described in United States. Letters Patent No. 2,240,955 for a High frequency wattmeter, issued to me. on

May 6th, 1941. Since the volumeand specific.

heat of; the piece of metal are known; the tem perature it will reach may be determined for any given power absorbed or, if heating of the piece to any particular temperature is desired. the amount of calories absorbed by it by the metal, piece being known from the power measurement, the time required for the piece to; reach the desired temperature can be calculated.

There are other ways in which-to determine the time necessary to reach the desired temperature, such as by experimentation, or other empirical methods.

Assuming that the desired time has been de-- termined, a time delay or timing switch I8 is,

viously described operation of the relay R3 in causing engagement between the contacts 45 and 46 to cut off the oscillations of the oscillater and subsequently quench the metal piece 10 or remove some from within the coil Ls.

Consequently the leads from the switch 18 are connected to the lead 41 and the contact a respectively. V

Another way of accomplishing; the same purposeis to use an auxiliary relay R3 in the circult and which willcpcrate at precisely the same time that the relay Ra operates to rematch the impedance of the oscillator. This is shown symbolically in Fig. 4 in which the contacts 18 and- 11 of the time delay switch 18 will engago at the same time that theCurie point is reached. Following this, and after the time chosen and set on the time delay switch has elapsed, the: circuit from the power source 19 to the plunger relay R6 will be closed causing the movement of the plunger 64, link 66, and plate 65, thereby causing the metal piece Hi to be removed from the coil L2. At the same instant that the time delay switch operates to actuate the plunger it will also operate through the leads 82 to cut oil the oscillator in the previously described manner. These leads are connected, to the lead 41 and the contact 43 and hence the time delay switch 18 also takes the place of the contacts I! and 46. 7

Other ways of accomplishing the same purpose should suggest themselves to those skilled in the art; such as the insertion of a time delay switch- 18 in the grid circuit 56' of the grid-controlled rectifier 51, or installing-such a switch in other convenient places of the circuit to cause similar operation.

It will be noted that the source of power symbolically shown in Fig. 4 is intended to represent any source of powercoextensive with the appanatus of the circuit, such as for example the A. C. source 36 shown in Fig. 2 and the switches. relays, etc., which are normally in the circuit but not shown for the purposes of clarity.-

While I have described methods and apparatus for'the heating of metals to points at and above the Curie point of magnetic transformation, it sometimes is-desirable to heat the metals to points below the Curie point, such as for example in annealing or the like.. The same method of cutting olf the oscillator as described above can be used to cut ofi the oscillator before the Curie point is reached. The meter 16 in the plate circuit measures the plate current and consequently also provides indications of the corresponding values of the power absorbed by the loadaccording to the curve P. Since the curve P has a definite relation to the temperature of the metal, the meter I6 can be calibrated as a temperature indicator. In the heating of pieces of metal which will heat up relatively slowly the metal piece may be kicked out of the coil at any desired temperature as indicated by the meter l8, by the manual actuation oirmechanical means well known in the art.

II it is desired that this be done automatically, it is possible to adjust the relays R1 and R3 to operate at any desired point of the P curve. For example, if it is desired that the metal piece be heated only up to a. temperature corresponding to the point X on the P curve, the relay R1 may be adjusted to operate at the maximum point ofthe curve, namely at P2 to prepare the circuits for the relays R2 and R4. for operation by therelay R3. 'I'he'relay Ra having been previously adjusted for operation when the plate current reaches a value corresponding to the point X on the power absorption curve, these relays R2 and R4 will be operated to cut off the oscillator before the Curie point is reached and permitting the piece I0 to be dropped out ofythe coil L2 in the manner previously described, but not necessarily into a quenching bath.

- .Likewise, the operations of relays at various points of the curve, that is, at various temperatures, may be controlled by adjustment of the response characteristics thereof in well known manners, to cause the oscillator to be shut off atanyprecisepoint of the P curve desired. -It is obvious that my invention is applicable to. a wide variety of use, and entirely eliminates any of the disadvantages of other methods of induction heating. It is also obvious that many of the advantages and features of my invention are in no wise shown or suggested in other methods of andiapparatuses for induction heating. *It isbelieved that my invention, its mode of construction, arrangement, assembly, association of; parts, the mode of practicing the method thereof, and many of the advantages and features should be readily understood from the foregoing without;further description, and it should also be manifest that while a preferred embodiment of the invention has been shown and described for .-illustrative purposes, the details are neverthelesscapable of wide variation within the purview of my invention as defined in the appended-claims.

What Iclaimgnd desire to secure by Letters Patent of. the United States is:

1. Themethod of heat treating an object the electrical characteristics of which vary with temperature, which method comprises closely coupling an object to be heated with heating means to which h gh frequency energy is supplied ,from an electronic high frequency generator so that the impedance of the heating means varies proportionately with the, variation in electrical characteristics of the object, coupling said heating means to the generator of high frequency 1 electrical energy at substantially unity impedance ratio between the equivalent resultant impedance of the heating .means with the object cou' pled to'it and the internal impedance of the generator between the points of coupling, and varying the last coupling to maintain that impedance ratio substantially equal'to unity as the electrical characteristics of the object vary with temperature.

.;2. The method. of heat treating a metal object, which comprises inserting the metal in an ele'ctromagnetic field. closely coupling a coil creating the field to a branch of the tank circuit of a high frequency, self-controlled generator, setting the Q of the tank circuit to obtain a predetermined loss resistance across the points of coupling to thegenerator substantially equal to theeifective loss resistance of the coil with the metal in its electromagnetic field, and decreasing the -Q of the tankcircuit as the permeability of the; metal decreases to reestablish substantial equality between the increased effective loss re sistance of the coil with the metal in its field and thelcss resistance of the generator across resistance substantially equal to the internalresistance of the electronic tube generator.

4. In a heat treating apparatus, a high frequency generator having a frequency determining parallel resonant circuit, a heater constructed for close coupling to an object to be heated whereby the impedance of the coil varies proportionately to the temperature of the object when energized from said generator, and a circuit including said heater and connecting the heater in an untuned branch of the frequency determining resonant circuit as a relatively small portion only of the impedance of said branch.

5. In a heat treating'apparatus, a high frequency generator having a frequency determining resonant circuit, a heater for producing a high frequency heat providing electrical field when energized from said generator, an untuned circuit including said heater and coupled to the frequency determining resonant circuit, and meansresponsive to a change in impedance of the heater circuit for automatically increasing the tightness of the coupling as the impedance of the heater circuit increases.

6. In a heat treating apparatus, a self-excited oscillator including a frequency determining circuit, and electronic energy control means for supplying energy to said'circuit at its resonance frequency, an induction heater coil adapted to receive a metal charge to be heated and closely coupled to said frequency determining circuit to reflect an equivalent impedance into said circuit, the ratio of coupling being predetermined to provide a resultant circuit impedance substantially equal to the internal impedance of the electronic energy-supplying means, and means con trolled by said circuit for varying said coupling ratio to maintain the circuit impedance substantially equal. to the internal impedance of the electronic energy supplying means as the equivalent impedance reflected into said circuit varies with the temperature of the charge.

'7. A method of transferring energy from a class C electronic oscillator to a metal member. to heat the latter, which method comprises cou-. pling a heating means to the oscillator at a predetermined coupling ratio substantially equal to the ratio of the terminal impedance of the oscillator to the impedance of the device with the object in the electrical field thereof to match the coupled impedance to the terminal impedance of the oscillator, and changing the coupling ratioa predetermined amount when the plate current-of the oscillator decreases to'a predetermined value to substantially rematch the coupled impedance to the internal impedance of the oscillator.

8. A method of controlling a supply of energy from a high frequency electronic oscillator to a heating means, which method comprises coupling the heating means with the object to be heated in the electrical field thereof to the oscillator at a. coupling ratio predetermined to provide a given plate current in the oscillator, and varying the coupling ratio a predetermined amount upon predetermined variation of the plate current toeffect an opposite change in the plate:

current thereby to. increase the energy supplied bm the oscillator to the member. being heated.

9. In a. heat treating apparatus, a high frequency electronic tube generator, means including an induction heater coil forming with a metal member to be heated a load connected between output terminals of the generator to receive power from the generator for heating the metal member when in .the field: of the coil, said gen.- erator including a variable impedance coupler preset to match the impedance of the generator between, its output terminals to the equivalent impedance of the load between its input terminals, means responsive to the generator for varying said'impedance coupler automatically as the metal member on heating varies the equivalent impedance of the load to rematch the impedance of'Ithe generator between its output terminals to the changed equivalent impedance of the load between its input terminals, timing means connected tosaid. generator andpreset, to automaticallyirender said generator ineiTective tosupply energy'to the load, a predetermined time after initiation of the operation of said timing means, and; means controlled by the generator for initiatingoperation of said timing means simultaneously with the automatic rematching of the load to the generator.

- 10. Inaheat' treating apparatus, a high frequency oscillator circuit including electronic tube means; means for biasing the. electronic tube means. sufiiciently negative to cause the plate current to vary with. the load, an induction coil for heating a piece ofmetal in the field of the coil, means for adjustably coupling said coil to said oscillator circuit at a ratio such that the impedance of the oscillator circuit is matched to the resultant impedance of the induction coil whenthe piece of metal is in the field of said coil, 4

and. means responsive to' the plate current or voltage of the oscillator for adjusting said coupling means asthe resultantimpedance of the coil varies with the heating .of..the metal torematch the impedance of the oscillator circuit to the resultant impedance of the coil.

11. In a heat treating apparatus-an electronic tube oscillatorcircuit having the grid circuit of the electronic tube biased substantially beyond theplate current cut off so that the plate current varies: with the loss resistance of the: oscillator tank circuit, an induction heater coil adapted to have a. piece of metal inserted therein for heating thereby, means for. adjustably coupling said: coil to the oscillator tank circuit at a ratio such that the equivalent loss resistance reflected into the tank circuit with the metal to be heated in the coil plus the loss resistance of. the tank circuit is substantially equal to the internal impedance of the oscillator across the-tank circuit, and means controlled by thecurrent or voltage of the tank circuit for adjusting said coupling means upon a predetermined variation in the impedance of the. heater coil as the temperature of the metal varies to substantially rematchthe reflected loss resistance in the tank circuit plus the loss resistance otthe tank circuit toithe impedance of the oscillator across the tank' circuit. r

n 12, In a heat treating apparatus, an electronic tubeoscillatorhaving its grid circuitbiased sufli-' ciently to cause the plate current to vary with the lossresistanceof its tank circuit, an induction heater coil adapted to receive a piece'of metaLfonheating the-same, means for adiustablv coupling said, coil: to the tank. circuit. of: the m cillator at a coupling ratiosuchv that the; ion resistancerefiected into the tank circuit from'the heater coil with the metal to be heated therein is substantially. equal to the internal impedanm of the oscillator across said; tank circuit,1 said piece of metal on being heated increasing. and then decreasingin permeability as the tempera-- ture thereof rises, and thereby increasing and decreasing the plate current of the oscillator correspondingly, means responsive tothe plate current of the oscillator for adjusting said coupling means as the permeability of the metal decreases to rematch the loss resistance reflected into the tank circuit tothe internal impedance of the oscillator across the tank circuit, and means responsive to the plate current of the 0&- cillator. for preventing operation ofsaid coupling adjusting means during increase ofthe plate current-as the permeability of the metal increases. 1

13. Ina high frequency-thermionic generating system having a high frequency, internal 1mpedance between the input terminals of its tank circuit, comprising an induction heater for heating a metal member when'in the field of the coil, saidheater coil being connected in shunt to a predetermined fraction of the turns of the inductance coil of the tank circuit to match there sultant impedance of the tank circuit to the internal impedance of the system between saidtan! circuit input terminals, and means controlledby the tank circuit for varying the shunt connection between the heater coil andtheinductance coil of the tank circuit to rematch the resultant impedance of saidtankcircuit tothe internal impedance of the system between theinput termi I nals of the tank circuit as the metal membersin being heated is brought to a temperature substantially approximating the Curie point of magnetic transformation.

14. In a-heat treating apparatus, an electronic oscillator circuit inciuding a tank circuit, an induction heater coil for heating a metal charge in the held of the coil;- means for adjustably and closely coupling'said heater coil to said tank circuit to reflect an equivalent loss resistance into the tank circuit, said coupling means being adjusted to match theimpedance oi. the tank circuit with said equivalent loss resistance to the internal-impedance 0i the'oscillator circuit across the tank circuit input terminals, a coil coupled to said tank circuit, a first relay energized by said last-mentioned coil, means including a secand relay controlled by the current supplied'to the tank circuit, means controlled by the first relay for changing said coupling means to rematch the impedance of the tank circuit with the loss resistance reflected thereinto from the heater coil to the internal impedance of the oscillator circuit across the tank circuit input terminals as the permeability of the metal charge decreases with increasing temperature, and means controlled by the second relay for rendering said oscillator ineffective during changing of ,said coupling.

15. In a heat treating apparatus, an electronic oscillator circuit including a tank circuit, an induction heater coil for heating a metal chargev in the field of the coil, means for adjustably and closely coupling said heater coil to said tank circuit to reflect an equivalent loss resistance into the tank circuit, said coupling means being adjusted to match the impedance of the tank circuit with said equivalent loss resistance ins- $1.4m. mineral-i. 'n.

" relay energized by said to the internal impedance of the oscillator circuit across the tank circuit input terminals, a co-il coupled to said tank circuit, a first last-mentioned coil, means including a second relay controlled by the current supplied by the tank circuit, means controlled by the first relay for changing said coupling means to rematch the impedance of the tank circuit with the loss resistance reflected thereinto from the heater coil to the internal impedance of the oscillator circuit across the 10 pling.

EUGEN MIT'IEIMANN. 

